1. Field of the Invention
The present invention relates in general to electrical switches, and, more particularly, to an apparatus and method for determining whether a switch, for use in an HVAC system, is in an electrically closed state or an electrically open state.
2. The Prior Art
In indoor environmental conditioning systems for spaces, such as HVAC (heating, ventilating and air conditioning) systems for residences and commercial spaces, external switches, such as automatic safety switches, are frequently coupled to control units. For example, HVAC systems typically employ such automatic safety switches as high pressure sensor switches, low pressure sensor switches, and flame rollout sensor switches. Each of these switches will automatically transition from a normally closed state to an open state upon the detection of an unsafe condition, such as a high pressure condition, a low pressure condition, or a flame exiting the combustion chamber, respectively. Generally, it is desirable for the state of these automatic, external safety switches to be sensed by an HVAC controller. For example, an HVAC controller may include a visual display, or other signalling circuitry, in order to provide an indication of the present state of these safety switches. Moreover, an HVAC controller may take certain actions, such as the institution of diagnostic or shut-down procedures, upon sensing a transition of the state of an external safety switch.
Often, safety switches are positioned at a remote location, relative to the HVAC controller. For example, an HVAC controller may be located proximate a furnace, which, for example, may be located in a basement of a building; while an associated automatic safety switch may be located proximate a compressor within an air conditioning unit, which, for example, may be located on the roof of the same building.
One type of prior art system for detecting the state of a switch is shown in FIG. 1 of the present application. In such prior art systems, a status conductor, coupled proximate a terminal of a safety switch, or a group of safety switches wired in series, is routed back to the HVAC controller. Voltage sensing circuitry within the HVAC controller detects whether a voltage is present or absent at the sensed location proximate the safety switch, and, accordingly, whether the safety switch, or any switch within the group of safety switches wired in series, is in a closed or open state. While the prior art system shown in FIG. 1 provides a means of detecting the state of an external switch, additional switch status conductors between the sensed switch and the status sensing circuitry are necessitated. As a result, cabling costs are increased, especially when, for example, the sensed automatic safety switches are located an appreciable distance from the associated HVAC controller. Moreover, the presence of the additional status signal conductor creates an additional failure mechanism for the system.
Another prior art system for determining the status of an external safety switch in an HVAC environment, is disclosed in U.S. Pat. No. 5,372,120 to Swilik, Jr. et al. This reference discloses coupling an external safety switch between the power supply and return terminals of a controller unit. The return terminal of the controller unit is also coupled to a microprocessor, which senses the presence or absence of a return voltage (as opposed to current flow), in order to determine whether the external safety switch is in an open or closed state. While this configuration does not require the additional, external status conductor shown in FIG. 1, it does require that the controller unit's return terminal be isolated from system ground.
Accordingly, it is desirable to provide an apparatus and method for determining a state of a switch by sensing current flow (as opposed to voltage), for use in an HVAC system, wherein an additional external conductor is not required in order to sense the state of the switch; and wherein a terminal of the sensed safety switch may be coupled to system ground.